In the manufacture of commercial paper, wood pulp is chemically treated to prepare it for further processing steps. Such pulping treatment and processing is typically performed in pulp treatment towers. The raw or partially treated wood pulp and chemical mixture will conventionally be fed into one end of the tower and migrate to the other end while being treated with such chemicals as bleaches, acids, bases and the like, so as to modify the color and fiber chemical and physical characteristics and consistency of the pulp to produce a product suitable for paper formation. One type of pulp treatment tower that is widely used is a tower wherein the raw or partly treated wood pulp is fed into the bottom of the tower and migrates therein to the top of the tower, where it is withdrawn for further processing. U.S. Pat. No. 5,015,335, granted May 14, 1991 to C. E. Green is a pulp stock bleach tower that operates in an upflow manner, the pulp and bleach mixture being fed into the lower end of the tower and discharged from the upper end thereof.
In the ideal bottom-to-top pulp treatment tower, the pulp will continuously flow in an even stream from the entry point to the discharge point, so that every aliquot of pulp in the discharge stream will have spent substantially the same amount of time in the tower, and will have been treated or chemically altered to the same extent. Thus, the ideal pulp treatment tower would produce a homogeneously treated discharge stream. In reality, however, pulp stock flowing through an upflow pulp treatment tower does not necessarily display even and steady flow characteristics. The pulp will often flow through the tower along faster and slower moving paths, with the pulp stock located adjacent the tower perimeter moving very slowly, and with the pulp stock located more inwardly of the tower frequently flowing along faster moving channels, much like a river, wherein the water at the banks will typically move more slowly than the water in the middle of the stream. The aforesaid pulp channels will also frequently meander from the inlet to the outlet of the tower. The resultant pulp discharge will therefore consist of fractions which have resided in the tower for longer periods of time than other fractions. Even when such a heterogeneous mixture is blended, the result is a product which is not as desirable as a homogeneously treated mixture. Scrapers may be installed in the top of the pulp treatment tower to mechanically control pulp flow and move the pulp stream toward the discharge pipes of the tower, but such scrapers are expensive and difficult to install in existing towers, and are generally operable to create only a tangential discharge flow pattern in the top of the tower. Such scrapers may not help in controlling pulp flow in lower portions of the tower.